The ultimate goal of this research is to define the human CTL response to leukemia by utilizing purified and well-characterized antigens, cloned CTL lines and monoclonal antibodies to functional T-cell subsets. Initial studies in murine xenogeneic, allogeneic and syngeneic CTL model systems demontrate that H-2 and HLA antigens can be isolated from the cell surface and retain CTL-stimulating activity. These results suggest that the antigenic requirements for the stimulation of human CTLs can be approached in a similar manner. We are continuing to define the human cytolytic T lymphocyte response by using monoclonal antibodies, cloned T-cell lines and purified HLA antigens incorporated into liposomes. Recently, a series of new monoclonal antibodies was generated that defines antigens involved in the human CTL target interaction. Three cell surface antigens associated with the cytolytic T lymphocyte (CTL)-target cell interaction were identified by generation of monoclonal antibodies (mAbs) against OKT4+, HLA-DR-specific CTL and selecting for inhibition of cytolysis in a 51Cr-release assay. These mAbs block cytolysis by both OKT4+ and OKT8+ CTL and the proliferative responses to PHA and the mixed lymphocyte response (MLR). LFA-1 is an antigen widely distributed on lymphoid tissues and is composed of two polypeptides of 177,000 and 95,000 Mr on all cell types studied. Anti-LFA-1 mAbs block NK cell-mediated cytolysis in addition to T lymphocyte-mediated cytotoxicity and proliferation. LFA-2 (Mr = 55,000 to 47,000), a determinant on the sheep red blood cell receptor, is expressed by T cells but not B cells and appears specific for T-cell functions. LFA-3 (Mr = 60,000) is a widely distributed antigen present on both hematopoietic and nonhematopoietic tissues and appears only to be involved in T-cell functions. Monoclonal antibodies to LFA-1 and LFA-2 inhibit function by binding to effector cell surface molecules while anti-LFA-3 mAbs appear to block by binding to the target cells. Together with previously described molecules, LFA-1, LFA-2 and LFA-3 demonstrate the complexity of CTL-mediated cytotoxicity at the molecular level.